WASHINGTON — White House hopes for stopping a congressional challenge to the Iran nuclear deal and sparing President Barack Obama from using a veto suffered a blow Friday when a key Senate Democrat announced his opposition.

The setback came in the announcement from Sen. Ben Cardin of Maryland, top Democrat on the Foreign Relations Committee, that he opposes the deal, which he said “legitimizes Iran’s nuclear program.”

Cardin’s move doesn’t affect the ultimate outcome for the international accord to curb Iran’s nuclear ambitions in exchange for relief from economic sanctions. The White House already clinched the necessary Senate votes to ensure that even if Obama ends up having to veto a disapproval resolution set for a vote next week, his veto would be upheld.

But with that support in hand and more piling up, the White House and congressional backers of the deal had begun aiming for a more ambitious goal: enough commitments to bottle up the disapproval resolution in the Senate with a filibuster, preventing it from even coming to a final vote.

With Cardin’s announcement, that goal remains in reach, but it will be tougher to attain.

“This is a close call, but after a lengthy review, I will vote to disapprove the deal,” Cardin wrote in an opinion piece in The Washington Post. “After 10 to 15 years, it would leave Iran with the option to produce enough enriched fuel for a nuclear weapon in a short time,” he wrote.

Cardin made his announcement as Obama met at the White House with King Salman of Saudi Arabia, in part to offer assurances that the deal signed by the U.S., Iran, Britain, France, Germany, China and Russia comes with the necessary resources to help check Iran’s regional ambitions. Saudi officials have cautiously supported the deal but are worried about enforcement and whether an Iranian government flush with cash will wreak havoc throughout the Middle East.

Before the meeting began, Obama told reporters in the Oval Office that the leaders would “discuss the importance of effectively implementing the deal to ensure that Iran does not have a nuclear weapon, while counteracting its destabilizing activities in the region.”

Later, the White House issued a statement saying that the Saudi ruler had “expressed his support” for the agreement.

With all but a handful of Senate Democrats already stating their positions – and only two opposed to the deal – Cardin was the critical outstanding vote. In addition to serving as top Foreign Relations Committee Democrat, he was an author of legislation providing for congressional review of the Iran deal. As a leading Jewish Democrat, he was also under strong pressure from segments of the Jewish community to turn down the deal, which is ardently opposed by Israel.

Cardin’s announcement came moments after Colorado Democratic Sen. Michael Bennet announced that he would back the deal. Bennet, who is up for re-election next year in a battleground state, said the agreement is flawed but represents an important step toward the objectives of preventing Iran from attaining a nuclear weapon, ensuring Israel’s security and avoiding war in the Middle East.

Bennet’s support put backers of the agreement just three votes shy of the 41 they would need to filibuster the resolution and block it from passing. But Cardin’s opposition could be enough to prevent those three additional votes from emerging. Only five senators have yet to announce where they stand: Richard Blumenthal of Connecticut, Maria Cantwell of Washington, Joe Manchin of West Virginia, Ron Wyden of Oregon and Gary Peters of Michigan.

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